It uses the default image for when an image isn't posted. Same is used in official apps, like Downloads. That's how it is:
{{if $file->primary_screenshot}}
{template="thumbImage" group="global" app="core" params="$file->primary_screenshot_thumb, $file->name, 'large', '', 'view_this', $file->url()"}
{{else}}
<div class='ipsPos_left ipsNoThumb ipsThumb ipsThumb_large'></div>
{{endif}}
You need to see in the CSS (core) which image it is in the ipsNoThumb class.